Optical disks such as CDs (compact disks), DVDs (digital versatile disks), optical card memories, and the like are used as an optical information recording medium.
In order to achieve a large capacity of the recording information, Non-patent Literature 1 discloses an information recording medium to be recorded by using a two photon absorption phenomenon in which a plurality of recording layers 101 are three-dimensionally laminated one on another as shown in FIG. 13.
The information recording medium 121 includes a transparent glass substrate 104 and recording layers 101a-101d made of an urethane-urea copolymer material which is a photon-mode recording material, and further includes intermediate layers 101a-102c made of PVA (polyvinyl alcohol) film and PMMA (poly(methyl methacrylate)). The recording layers and the intermediate layers are laminated one on another on the transparent substrate 104.
Upon recording, recording light 122a having a large peak-power of a wavelength of 0.790 Mm irradiated from a titanium-sapphire laser as a recording light source 120a passes through a beam splitter 118a, its beam radius is expanded by a beam expander 123, further passes through a beam splitter 118b, and converges (convergent light 107) to a desired recording layer 101c of a three-dimensionally recordable/reproducible multi-layered information recording medium 121 by an objective lens 106. When the convergent light 107 is focused, only a portion of light having a high optical power density (focal point and therearound) will be absorbed as if a wavelength of the recording light becomes a half length by a two photon absorption process, thereby forming a recording pit 105. Therefore, even if the number of the recording layers increases, the other recording layers are still substantially transparent to the recording light and two photon absorption occurs only at a predetermined recording position. As a result thereof, the recording light is prevented from attenuation to enable satisfactory recording at a lower recording layer positioned away from the objective lens 106.
On the other hand, upon reproduction, the reproduction light 122b having a small peak-power of a wavelength of 0.6328 μM irradiated from a He—Ne laser of a reproduction light source 120b is converged (convergent light 107) in a similar manner onto the recording pit 105 of the desired recording layer 101c by the objective lens 106. Then, the reflected light is bent in a Y axis direction by means of a beam splitter 118b, converged by a detection lens 111 to pass through a pin hole 114 positioned at a focal point of the detection lens 111, and thereby detected by a light detector 119, resulting in signals being reproduced.
In an optical disk recording/reproducing apparatus using a conventional single photon absorption phenomenon, a focus servo is performed, upon recording or reproducing, so as to have recording light or reproduction light from a light source converge precisely onto a disk through the objective lens. Although it is not discussed in the Non-patent Literature 1, it is preferable that the focus servo is performed when the recording pit 105 is reproduced in an information recording medium 121 disclosed in Non-patent Literature 1. More specifically, for example, when recording light 122a or reproduction light 122b is irradiated onto the information recording medium 121, if reflected light having a certain intensity can be acquired from the recording layer 101c, the recording light 122a or the reproduction light 122b can be precisely converged onto the desired recording layer 101c by using the reflected light as a focus servo light.
However, according to a study of inventors of the present invention, when the above-described focus servo is performed on the information recording medium 121 disclosed in Non-patent Literature 1, it is considered that there will be the following problems.
That is, in order to precisely converge the recording light or the reproduction light onto the desired recording layer 101c upon recording or reproducing of the information recording medium 121, focus servo reflected light of a constant intensity is required at each of the recording layers 101 with regard to both of the wavelength of the recording light and the wavelength of the reproduction light. The larger the light intensity of the reflected light becomes, the easier the focus servo becomes.
An information recording medium 121 is provided with a plurality of recording layers 101 in a multi-layered manner in order to form a recording pit three-dimensionally. The recording light and the reproduction light are reflected and absorbed not only by the desired recording layer 101c but also by each of the recording layers 101a-101d. Therefore, due to the reflection and absorption by each of the recording layers 101a-101d, the light intensity of the recording light and the reproduction light will be lowered before they reach the lowermost layer farthest from the objective lens (101d in FIG. 13). Especially, if the number of the recording layers becomes equal to or more than 4 layers, the light intensity will be remarkably lowered. Accordingly, if one attempts to obtain a sufficient reflected light for the sake of the focus servo of each of the recording layers by using the recording light or the reproduction light, transmitted light of the recording light or the reproduction light will decrease, and therefore, such a problem will occur that a good recording pit will not be formed up to the lowermost recording layer in a recording process which requires a large amount of light such as required in the two photon absorption process. Especially, in the case of the two photon absorption recording, the recording sensitivity remarkably decreases in accordance with lowering of the amount of recording light upon, in comparison with a single photon absorption recording. That is, in the case of the single photon absorption recording, the recording sensitivity is proportional to the light intensity, whereas in the case of the two photon absorption recording, the recording sensitivity is proportional to a square characteristic of the light intensity. In other words, for example, if the amount of the recording light is multiplied by 0.5, the recording sensitivity is multiplied by 0.5 in the single photon absorption, whereas the recording sensitivity is multiplied by 0.25 in the two photon absorption recording, which is a square of the recording sensitivity in the single photon absorption. Consequently, it is difficult for the conventional information recording medium of Non-patent Literature 1 to secure a sufficient amount of light until it reaches the lowermost recording layer upon recording while securing the focus servo reflected light. Non-linear recording necessitates a semiconductor laser having a high peak-power of, for example, more than several hundreds mW to 1 W as a recording light source, such that there is almost no actual room for power adjustment of the light source actually to the higher output power. As such, when the lower recording layer is recorded in order for the recording pit to be three-dimensionally recorded in a recording unit, it is often difficult to raise the power of the recording light source to more than the power for recording an upper recording layer.
[Non-patent Literature 1]
Yoshimasa KAWATA: Three-dimensional Optical Memory using a Femto-second Laser, OPTRONICS No. 11 pp. 138-142 (2001)